1. Field of the Invention
This invention pertains to the art of manufacturing plys of structural fabric characterized by the presence of parallel, obliquely biased yarns of high modulus stitched together into a uniform ply, the ply having a high uniform density. More specifically, the invention provides a method for making plys of structural fabric of high yarn density, the yarns being parallel, biased and stitched together in a ply of uniform density through a weft-insertion process.
2. Description of the Prior Art
The increasing demand for composite materials comprised of fabrics produced from structural yarns of high modulus and impregnated with a cured resin, wherever high strength and stiffness and low weight is required, such as the aerospace industry, has placed a premium on process technology capable of providing a large volume of such structural fabrics at a reasonable cost, and of meeting exceedingly high specifications, such as those observed in the aerospace industry. One common type of structural fabric is a non-woven, stitched or knitted fabric, that is comprised of a plurality of plys of structural yarns. Because of the dynamics of stress and strain, and the loads applied to the structural fabrics, it is desirable, in many applications, to have the yarns present, in each individual ply, in a precise parallel array. Additionally, control over the density of yarns in the fabric, and control over the uniformity of density in the fabric, is critical. Frequently, it is desired to have a majority of the plys laid in at a bias to the fabric.
One of the most productive and cost-efficient methods of providing plys of structural yarns is the weft-insertion process, where a plurality of yarns is directed back and forth across a space defined by two advancing rows of hooks or retaining elements, the yarns being retained at each crossing by those retaining elements. As the yarns are retained by these elements, the hooks are advanced into a stitching machine, where stitching yarns are applied across the structural yarns. A single ply of fabric, with yarns oriented in a direction of 90.degree. to the direction of the fabric, the yarns being relatively widely spaced is produced. The yarns are widely spaced due to the need to pass the yarns and carrier elements between the hooks, or otherwise retain them. Additionally, it is necessary to maintain this spacing to ensure the knitting needle can pass between adjacent yarns without engaging additional yarns or portions of yarns thereby producing a flaw in the fabric. Density of the fabric is commonly increased by making the infeed of laid-in yarns to the stitching machine higher than the output of stitched fabric. The yarns thus tend to "pile-up" in the machine, increasing density. Methods employing a differential between infeed and output are not applicable to biased or multi-ply fabrics or any other situation where infeed and output must be equal.
The chief drawback to such systems is that the best approximation of parallelity that can be made when a biased fabric, a multiple ply fabric or any other fabric produced where density on infeed is equal to density of output is what is termed a "cross-over" weft, where the structural yarns may vary from actual parallelity through a broad range. This cross-over is an inherent feature of such processes, as the shuttle which directs or carries the yarns reaches the end of its traverse, and returns, cross-over the yarns at that point. Such systems are featured in U.S. Pat. No. 3,756,893 which is particularly directed to providing a process for incorporating a multiple of layers in a single stitching operation, the yarn being oriented at any given direction.
Attempts have been made to overcome this inherent drawback in weft-insertion apparatus.
Thus, U.S. Pat. No. 4,325,999, in FIG. 11, columns 13 and 14, illustrates a method by which a 2-layer non-structural fabric can be produced, wherein each of the yarns in each layer is parallel, the yarns in respective layers being aligned at 90.degree. to each other and 45.degree. with respect to the long axis of the fabric. It should be noted that the apparatus requires the yarns to actually pass through spaces between the pins or retaining elements on the conveyor, and then turn around and pass back across those elements. The resulting fabric is therefore necessarily of low density, and not suitable for the above-described applications. A different approach is described in Canadian Patent No. 912,296, wherein the shuttle, which carries the yarn back and forth, carries the yarn through the retaining elements on the conveyors, and then moves at right angles to its basic traverse, or "racks", backwardly to positively engage the yarn around the retaining elements. While the Canadian Patent does not describe this feature, if racking motion of the shuttle is correctly controlled, this can result in the deposition of yarns that are parallel along their length. Such a result is described in British Patent No. 1,299,638. However, to achieve this goal, this apparatus also requires that the yarns travel between the retaining elements on each traverse.
While the requirement that the yarn travel between the retaining elements poses no problems for the production of fabrics which are not particularly dense, such as the designer fabrics of the Canadian Patent, an insurmountable obstacle is presented when making the high density structural fabric required for many technical applications such as the aerospace industry. When the yarn density in biased or multilayer fabrics is such that the yarn spacing must be less than about 0.3 inches and therefor the retaining rack, pin or element spacing must also be less than this value, problems occur.
A rather severe problem, particularly encountered with mechanisms racking the shuttle which is necessary to achieve parallelity, is the tendency of the yarns to fall between retaining elements other than those intended. As the yarn is not truly engaged until the shuttle is well on its way to the other end of its traverse, the yarn may slip into any of a number of spaces, causing many faults in the fabric. Additionally, overall, density is not likely to be uniform.
As a solution to this problem, a variety of yarn transfer mechanisms, or "rakes" have been developed, which receive the yarns from the yarn carriers, and then transfer them to the retaining elements when positive engagement is assured. Exemplary among them is U.S. Pat. No. 3,756,043. However, these rakes do not address the problem of achieving high density with parallelity.
In particular, these rake embodiments cannot be used to achieve high density because the spacing of transfer elements on the rake is required to be the same as the yarn density on the shuttle. At high densities, necessarily some yarns will be impaled by the transfer elements, rather than properly engaged. Upon movement of the rake, breakage of the yarns and resulting faults are likely to occur.
As a result, the only available, reliable method for providing high, uniform density structural fabric plies wherein the fibers are arranged in parallel is a single-end weft-insertion, where only a single yarn end is passed between the conveyors by the shuttle, or a machine where infeed and output density are not equal. The former process is capable of extreme reliability, but it is extraordinarily slow. The latter is limited to non-biased fabrics, those laid in at an angle of 90.degree. with respect to the fabric. As a result, neither is a commercially acceptable method for producing the desired fabrics.
A further problem presented by commercial demands is the need to provide a wide variety of fabrics having different, but uniform, densities through a single fabrication technology. Even within a single application, e.g. aerospace, there is a need to be able to provide fabrics of a wide range of densities but common appearance and manufacturing.
Accordingly, there continues to be a pressing need for the provision of a proces whereby high density structural fabrics, suitable for impregnation with a resin in a fiber form, can be made at a reasonable cost such that the biased yarns of the fabric are all in parallel array, and otherwise resemble the products of weft-insertion.